Single mode, optical fiber couplers are useful as signal distributors in multiple terminal systems such as those for bringing wideband services to consumers. One scheme for delivering such services involves telephone over passive networks, TPON, and an associated broadband version called BPON which serves up to 128 private users using a 20 Megabyte per second, Mb/s, time division multiplexed arrangement.
A known effective construction of a coupler involves two single mode fibers which are drawn and fused to form a twin fiber, biconical taper structure. One method of making such an optical fiber coupler comprises, first positioning a fiber bundle in a predetermined orientation and applying a tensile load to at least one of the fibers. At the same time, heat is applied to soften the fibers. The application of a tensile load causes necking down of each fiber into biconical shaped sections which fuse together because the fibers are softened. Fusing together of the biconical sections may be enhanced by twisting them while they are in a softened condition. As a further enhancement, the cladding of each fiber at the intended biconical section may be etched to a predetermined thickness prior to drawing and fusing.
According to another method, the fibers may be contained within a glass tube so as to promote intimate contact among biconical sections during fusion. The glass tube can comprise a quartz tube formed of complementary semicylindrical sections which are glued or fused together around the fiber bundles.
Receivers in a multiple terminal distribution system accommodate a particular power range, and respond poorly to signals either above or below the power range. A coupler for use in such a distribution system must couple output power uniformly in all outputs, for all outputs to attain output power within a prescribed power range. According to a publication, D. B. Mortimore and J. W. Arkwright, "Monolithic Wavelength-Flattened 1.times.7 Single-Mode Fused Coupler", Electron. Lett. 25, 606-607 (1989), have reported the fabrication of 1.times.7 single mode fused couplers by a method wherein 7 standard single-mode fibers are inserted into a capillary tube of Vycor, a trademark of Corning Incorporated, Corning N.Y. The fibers are arranged in a packed matrix structure with six output fibers surrounding a central fiber. The fibers are heated and tapered using standard, fused coupler fabrication equipment. During the heating and tapering process, optical power is launched into the central fiber, and the coupled power is monitored in each output fiber to achieve equal coupling of 14.3% to each output port. The Mortimore and Arkwright paper concludes that to obtain a wavelength flattened device, having equal power coupling, it is essential to control the degree of fusion of the coupler structure.